Memory device and method for estimating characteristics of multi-bit programming

ABSTRACT

Memory devices and/or methods that may estimate characteristics of multi-bit cell are provided. A memory device may include: a multi-bit cell array; a monitoring unit to extract a threshold voltage change over time value for reference threshold voltage states selected from a plurality of threshold voltage states corresponding to data stored in the multi-bit cell array; and an estimation unit to estimate a threshold voltage change over time values for the plurality of threshold voltage states based on the extracted threshold voltage change. Through this, it is possible to monitor a change over time of threshold voltages of a memory cell.

PRIORITY STATEMENT

This application is a continuation of and claims priority under 35U.S.C. §§120/121 to U.S. patent application Ser. No. 12/801,505, filedon Jun. 11, 2010, which is a continuation of and claims priority under35 U.S.C. §§120/121 to U.S. patent application Ser. No. 12/213,657,filed on Jun. 23, 2008, which claims priority under 35 U.S.C. §119 fromKorean Patent Application No. 10-2008-0002230, filed on Jan. 8, 2008, inthe Korean Intellectual Property Office (KIPO), the entire contents ofwhich are incorporated herein by reference.

BACKGROUND

1. Field

Example embodiments relate to methods of estimating characteristics ofmemory cells of memory devices. Also, example embodiments relate toapparatuses and/or methods that may estimate characteristics ofMulti-Level Cells (MLCs) (or Multi-Bit Cells (MBCs) of MLC memorydevices.

2. Description of Related Art

A single-level cell (SLC) memory device may store one bit of data in asingle memory cell. The SLC memory may be referred to as a single-bitcell (SBC) memory. The SLC memory may store and read data of one bit ata voltage level included in two distributions that may be divided by athreshold voltage level programmed in a memory cell. The programmedthreshold voltage may have a distribution within a certain range due toa fine electric characteristic difference between the SLC memories. Forexample, when a voltage level read from the memory cell is greater than0.5V and less than 1.5V, it may be determined that the data stored inthe memory cell has a logic value of “1”. When the voltage level readfrom the memory cell is greater than 2.5V and less than 3.5V, it may bedetermined that the data stored in the memory cell has a logic value of“0”. The data stored in the memory cell may be classified depending onthe difference between cell currents and/or cell voltages during thereading operations.

Meanwhile, a multi-level cell (MLC) memory device that may store data oftwo or more bits in a single memory cell has been proposed in responseto a need for higher integration of memory. The MLC memory device mayalso be referred to as a multi-bit cell (MBC) memory. However, as thenumber of bits stored in the single memory cell increases, reliabilitymay deteriorate and a read-failure rate may increase. To store ‘m’ bitsin a single memory cell, 2 m voltage level distributions may berequired. But, since the voltage window for a memory device may belimited, the difference in threshold voltage between adjacent bits maydecrease as ‘m’ increases, causing the read-failure rate to increase.For this reason, it may be difficult to improve storage density usingthe MLC memory device according to a conventional art.

Also, a portion of a charge generating the threshold voltage of the datain an SLC or an MLC may be discharged over time due to a leakage pathafter being programmed. The threshold voltage of the SLC or the MLC maybe changed by a discharge mechanism of a partial charge as time passesafter being programmed. Since accurate information about the thresholdvoltage in the MLC is required, accurately estimating a change of thethreshold voltage over time is very important.

SUMMARY

Example embodiments may provide apparatuses and/or methods that mayestimate changes over time of programmed threshold voltages ofmulti-level cells (or multi-bit cells).

Example embodiments may also provide estimation models that may estimatechanges over time of programmed threshold voltages of multi-level cells(or multi-bit cells).

Example embodiments may also provide apparatuses and/or methods that mayestimate changes over time of programmed threshold voltages ofmulti-level cells (or multi-bit cells) and may reduce errors whenreading multi-bit data.

According to example embodiments, a memory device may include: amulti-bit cell array; a monitoring unit to extract threshold voltagechange over time values of reference threshold voltage states selectedfrom a plurality of threshold voltage states corresponding to datastored in the multi-bit cell array; and an estimation unit to estimate athreshold voltage change over time values for the plurality ofnon-reference threshold voltage states based on the extracted thresholdvoltage change values.

According to example embodiments, a memory device may include: amulti-bit cell array; and a data detection unit to read a multi-bit cellof a first threshold voltage state using a first read voltage, and toread a multi-bit cell of a second threshold voltage state using a secondread voltage after data is programmed in the multi-bit cell array.According to example embodiments, the data detection unit may read themulti-bit cell of the first threshold voltage state using a third readvoltage, and read the multi-bit cell of the second threshold voltagestate using a fourth read voltage after a passage of time after readingthe multi-bit cell of the multi-bit cell array using the first readvoltage and the second read voltage. A difference between the fourthread voltage and the second read voltage may be greater than adifference between the third read voltage and the first read voltage.

According to example embodiments, a method of estimating acharacteristic of a multi-bit cell may include: selecting a plurality ofreference threshold voltage states from a plurality of threshold voltagestates corresponding to data stored in a multi-bit cell array;extracting threshold voltage change over time values for the pluralityof reference threshold voltage states; and estimating threshold voltagechange over time values for the plurality of non-reference thresholdvoltage states based on the extracted threshold voltage change values.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of example embodiments willbecome more apparent by describing in detail example embodiments withreference to the attached drawings. The accompanying drawings areintended to depict example embodiments and should not be interpreted tolimit the intended scope of the claims. The accompanying drawings arenot to be considered as drawn to scale unless explicitly noted.

FIG. 1 illustrates a threshold voltage change over time of multi-bitcells of a memory device;

FIG. 2 illustrates a method of estimating a characteristic of amulti-bit cell according to at least one example embodiment;

FIG. 3 illustrates a method of estimating a characteristic of amulti-bit cell according to at least one example embodiment;

FIG. 4 is a diagram illustrating a memory device according to at leastone example embodiment; and

FIG. 5 is a flowchart illustrating a method of estimating acharacteristic of a multi-bit cell according to at least one exampleembodiment.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Detailed example embodiments are disclosed herein. However, specificstructural and functional details disclosed herein are merelyrepresentative for purposes of describing example embodiments. Exampleembodiments may, however, be embodied in many alternate forms and shouldnot be construed as limited to only the embodiments set forth herein.

Accordingly, while example embodiments are capable of variousmodifications and alternative forms, embodiments thereof are shown byway of example in the drawings and will herein be described in detail.It should be understood, however, that there is no intent to limitexample embodiments to the particular forms disclosed, but to thecontrary, example embodiments are to cover all modifications,equivalents, and alternatives falling within the scope of exampleembodiments. Like numbers refer to like elements throughout thedescription of the figures.

It will be understood that when an element is referred to as being “on,”“connected to,” “electrically connected to,” or “coupled to” to anothercomponent, it may be directly on, connected to, electrically connectedto, or coupled to the other component or intervening components may bepresent. In contrast, when a component is referred to as being “directlyon,” “directly connected to,” “directly electrically connected to,” or“directly coupled to” another component, there are no interveningcomponents present. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

It will be understood that although the terms first, second, third,etc., may be used herein to describe various elements, components,regions, layers, and/or sections, these elements, components, regions,layers, and/or sections should not be limited by these terms. Theseterms are only used to distinguish one element, component, region,layer, and/or section from another element, component, region, layer,and/or section. For example, a first element, component, region, layer,and/or section could be termed a second element, component, region,layer, and/or section without departing from the teachings of exampleembodiments.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like may be used herein for ease of description todescribe the relationship of one component and/or feature to anothercomponent and/or feature, or other component(s) and/or feature(s), asillustrated in the drawings. It will be understood that the spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, and/or components.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which example embodiments belongs. Itwill be further understood that terms, such as those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andshould not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

Reference will now be made to example embodiments, which are illustratedin the accompanying drawings, wherein like reference numerals may referto like components throughout.

Generally, programming of a non-volatile memory may be performed bychanging a threshold voltage of a memory cell. The threshold voltage ofthe memory cell may denote data stored in the memory cell. The thresholdvoltage of the memory cell may have an error of a predetermined rangeand a distribution state.

After the data is stored in the non-volatile memory, the thresholdvoltage of the programmed memory cell may be changed by a mechanismincluding a charge loss in a floating gate and the like.

Example embodiments provide a memory device and a monitoring method formonitoring a change of the threshold voltage of the memory cell, and amemory programming apparatus and method that may minimize area overheadwhen verifying an error of the data stored in the memory cell.

FIG. 1 illustrates threshold voltage change over time of multi-bit cellsof a memory device.

Referring to FIG. 1, a horizontal axis denotes threshold voltages, and avertical axis denotes a number of multi-bit cells having the thresholdvoltages.

As described above, the threshold voltages of the memory cells form adistribution state of a predetermined range. A distribution state 110 ofthe threshold voltages denotes a distribution of the threshold voltagesof the multi-bit cells in which data “11” is programmed. A distributionstate 120 denotes a distribution of the threshold voltages of themulti-bit cells in which data “10” is programmed. A distribution state130 denotes a distribution of the threshold voltages of the multi-bitcells in which data “00” is programmed. A distribution state 140 denotesa distribution of the threshold voltages of the multi-bit cells in whichdata “01” is programmed.

The distribution state 110, the distribution state 120, the distributionstate 130, and the distribution state 140 denote distributions of thethreshold voltages of the multi-bit cells immediately after the data areprogrammed.

When an amount of time passes after the data is programmed, thethreshold voltages of the multi-bit cells may be changed by a mechanismincluding a charge loss in a floating gate and the like.

There are several causes of changing the threshold voltages of themulti-bit cells from the threshold voltages immediately after beingprogrammed, and it is known that the above-described charge loss mayreduce the threshold voltages of the multi-bit cells.

A distribution state 141 denotes an example of a distribution of thethreshold voltages that the multi-bit cells in which data “01” isprogrammed may have after a passage of an amount of time.

A distribution state 131 denotes an example of a distribution of thethreshold voltages that the multi-bit cells in which data “00” isprogrammed may have after a passage of the amount of time.

A distribution state 121 denotes an example of a distribution of thethreshold voltages that the multi-bit cells in which data “10” isprogrammed may have after a passage of the amount of time.

It is known that the threshold voltage change by the charge lossmechanism may increase as the threshold voltage increases. A differencebetween the distribution state 141 and the distribution state 140 may begreater than a difference between the distribution state 131 and thedistribution state 130, and a difference between the distribution state131 and the distribution state 130 may be greater than a differencebetween the distribution state 121 and the distribution state 120.

FIG. 2 illustrates a method of estimating a characteristic of amulti-bit cell according to at least one example embodiment.

Referring to FIG. 2, a threshold voltage change amount with respect tothree reference threshold voltage states is measured.

In the example embodiment illustrated in FIG. 2, it is assumed thateight different distribution states exist. However, it will beunderstood that the methods according to example embodiments may beapplied to any number of distribution states for example, 2, 4 or 16.Since each distribution state denotes a distribution state of thethreshold voltages, this is referred to as a threshold voltage state.

The estimation method may include selecting three reference thresholdvoltage states, and measuring a threshold voltage change over time valuefor each of the selected reference threshold voltage states.

The estimation method may include selecting a first threshold voltagestate as a first reference threshold voltage state, selecting a fourththreshold voltage state as a second reference threshold voltage state,and selecting a seventh threshold voltage state as a third referencethreshold voltage state.

The estimation method may include monitoring threshold voltages ofmulti-bit cells corresponding to the first reference threshold voltagestate, and finding a first change amount 210 over time of the thresholdvoltages.

The estimation method may include monitoring threshold voltages ofmulti-bit cells corresponding to the second reference threshold voltagestate, and finding a second change amount 220 over time of the thresholdvoltages.

The estimation method may include monitoring threshold voltages ofmulti-bit cells corresponding to the third reference threshold voltagestate, and finding a third change amount 230 over time of the thresholdvoltages.

The estimation method may include estimating, based on the first changeamount 210, the second amount 220, and the third change amount 230 ofthe threshold voltages, a change amount over time for each of theremaining five threshold voltage states excluding the three referencethreshold voltage states.

The estimation method may include estimating a change amount of a secondthreshold voltage state 211, a change amount of a third thresholdvoltage state 212, a change amount of a fifth threshold voltage state221, a change amount of a sixth threshold voltage state 222, and achange amount of an eighth threshold voltage state 231 based on thefirst change amount 210, the second change amount 220, and the thirdchange amount 230.

The estimation method may include generating a characteristic changemodel to estimate the threshold voltage change amount of each ofnon-reference threshold voltage states using a curve fitting schemebased on the first change amount 210, the second change amount 220, andthe third change amount 230.

The estimation method may include predicting the threshold voltagechange amount of each of the threshold voltage states using thecharacteristic change model acquired experimentally in advance.According to example embodiments, the estimation method may includespecifying a coefficient of the characteristic change model, which maybe acquired in advance using the curve fitting scheme, based on thefirst change amount 210, the second amount 220, and the third changeamount 230.

The characteristic change model may be a linearly increasing model or alinearly decreasing model based on a value of the threshold voltage. Thecharacteristic change model may be a non-linear function model includinga quadratic function, a cubic function, and the like, that non-linearlyincreases or decreases based on the value of the threshold voltage.

The characteristic change model may be a multinomial function modelbased on the value of the threshold voltage. The characteristic changemodel may be an exponential function based on the value of the thresholdvoltage.

An effort to increase a number of bits of the data stored in a singlemulti-bit cell for increasing a data storage density continues. As thenumber of bits of the data stored in the single multi-bit cellincreases, a number of threshold voltage states generated in a multi-bitcell array may increase exponentially.

Specifically, two threshold voltage states may be necessary for storing1-bit data in a single cell, however, four threshold voltage states maybe necessary for storing 2-bit data in the single cell, and 16 thresholdvoltage states may be necessary for storing 4-bit data in the singlecell.

As the number of threshold voltage states increases, a significant timemay be required for measuring the change amount of all threshold voltagestates, and a configuration for this may become complex. According toexample embodiments, a method of estimating a characteristic of amulti-bit cell may include selecting only some threshold voltage statesas reference voltage states rather than all threshold voltage states asthe reference threshold voltage states, measuring the change amount ofthe selected reference threshold voltage states, and estimating thechange amount of all threshold voltage states based on the measuredchange amount.

The estimation method may reduce complexity of the configuration, reducea time of reading the multi-bit cells, and reduce a number of reads ofthe multi-bit cells.

FIG. 3 illustrates a method of estimating a characteristic of amulti-bit cell according to at least one example embodiment.

Referring to FIG. 3, a threshold voltage change amount with respect tothree reference threshold voltage states may be measured.

In the example embodiment illustrated in FIG. 3, it is assumed thateight different distribution states exist. However, it will beunderstood that the methods according to example embodiments may beapplied to any number of distribution states for example, 2, 4 or 16.

The estimation method may include selecting three reference thresholdvoltage states, and measuring a threshold voltage change over time valuefor each of the selected reference threshold voltage states.

The estimation method may include selecting a second threshold voltagestate as a first reference threshold voltage state, selecting a fifththreshold voltage state as a second reference threshold voltage state,and selecting an eighth threshold voltage state as a third referencethreshold voltage state.

The estimation method may include monitoring threshold voltages ofmulti-bit cells corresponding to the first reference threshold voltagestate, and finding a first change amount 310 over time of the thresholdvoltages.

The estimation method may include monitoring threshold voltages ofmulti-bit cells corresponding to the second reference threshold voltagestate, and finding a second change amount 320 over time of the thresholdvoltages.

The estimation method may include monitoring threshold voltages ofmulti-bit cells corresponding to the third reference threshold voltagestate, and finding a third change amount 330 over time of the thresholdvoltages.

The estimation method may include estimating, based on the first changeamount 310, the second amount 320, and the third change amount 330 ofthe threshold voltages, a change amount over time of the remaining fivethreshold voltage states excluding the three reference threshold voltagestates.

The estimation method may include estimating a change amount of a firstthreshold voltage state 311, a change amount of a third thresholdvoltage state 321, a change amount of a fourth threshold voltage state322, a change amount of a sixth threshold voltage state 331, and achange amount of a seventh threshold voltage state 332 based on thefirst change amount 310, the second change amount 320, and the thirdchange amount 330.

The estimation method may include generating a characteristic changemodel to estimate the threshold voltage change amount of each ofthreshold voltage states using a curve fitting scheme based on the firstchange amount 310, the second change amount 320, and the third changeamount 330.

In at least the example embodiment illustrated in FIG. 3, the estimationmethod may include generating a linearly increasing or decreasingcharacteristic change model according to a value of the thresholdvoltage based on the first change amount 310, the second change amount320, and the third change amount 330.

FIG. 4 is a diagram illustrating a memory device 400 according toexample embodiments.

Referring to FIG. 4, according to example embodiments, the memory device400 includes a multi-bit cell array 410, a monitoring unit 420, anestimation unit 430, and a detection unit 440.

The monitoring unit 420 may extract threshold voltage change over timevalues for reference threshold voltage states selected from a pluralityof threshold voltage states corresponding to data stored in themulti-bit cell array 410.

The estimation unit 430 may estimate a threshold voltage change overtime of the plurality of threshold voltage states based on the extractedthreshold voltage change.

The detection unit 440 may reads multi-bit data stored in the multi-bitcell array 410 based on the estimated threshold voltage change.

The monitoring unit 420 may extract a first threshold voltage changeover time of a first reference threshold voltage state and a secondthreshold voltage change over time of a second reference thresholdvoltage state. According to example embodiments, the estimation unit 430may estimate the threshold voltage change over time values of theplurality of threshold voltage states by performing a linearapproximation based on the first threshold voltage change and the secondthreshold voltage change.

The monitoring unit 420 may extract a first threshold voltage changeover time of a first reference threshold voltage state, a secondthreshold voltage change over time of a second reference thresholdvoltage state, and a third threshold voltage change over time of a thirdreference threshold voltage state. According to example embodiments, theestimation unit 430 may estimate the threshold voltage change over timevalues of the plurality of threshold voltage states by performing aquadratic function approximation based on the first threshold voltagechange, the second threshold voltage change, and the third thresholdvoltage change.

The estimation unit 430 may estimate, based on the extracted thresholdvoltage change values and a number of reads of the data from themulti-bit cell array 410, the threshold voltage change over time valuesof the plurality of threshold voltage states.

The estimation unit 430 may estimate, based on the extracted thresholdvoltage change values and a number of writes and erasures of the datafrom the multi-bit cell array 410, the threshold voltage change overtime values of the plurality of threshold voltage states.

The estimation unit 430 may estimate, based on the extracted thresholdvoltage change values and an elapsed time after the program of the datain the multi-bit cell array 410, the threshold voltage change over timevalues of the plurality of threshold voltage states.

The estimation unit 430 may generate error information by error controlcodes (ECC) decoding of the data read from the multi-bit cell array 410,and estimate the threshold voltage change over time values of theplurality of threshold voltage states based on the extracted thresholdvoltage change values and the error information.

The monitoring unit 420 may extract the threshold voltage change overtime values of the reference threshold voltage states by adjusting readvoltage levels with respect to the multi-bit cell array 410. Accordingto example embodiments, the memory device 400 may store, in themulti-bit cell array 410, a number of multi-bit cells corresponding toeach of the reference threshold voltage states. The monitoring unit 420may extract the threshold voltage change over time values of thereference threshold voltage states based on the number of multi-bitcells corresponding to each of the reference threshold voltage states,the number being stored in the multi-bit cell array 410.

The monitoring unit 420 and the detection unit 440 may be embodied as aperipheral circuit around the multi-bit cell array 410. The monitoringunit 420 may detect the threshold voltages of the multi-bit cells of themulti-bit cell array 410 using variable read voltage levels. Themonitoring unit 420 may compare the variable read voltage levels and thethreshold voltages of the multi-bit cells and detect a number ofmulti-bit cells having the threshold voltages higher than each of thevariable read voltage levels.

Referring to FIG. 3 again, an example embodiment of FIG. 4 is described.A memory device 400 may store, in the multi-bit cell array 410, a numberof first multi-bit cells programmed to correspond to a first referencevoltage state, a number of second multi-bit cells programmed tocorrespond to a second reference voltage state, and a number of thirdmulti-bit cells programmed to correspond to a third reference voltagestate as meta data.

The monitoring unit 420 may store information about a first read voltagelevel that may discriminate the first reference voltage stateimmediately after data is programmed, from the first threshold voltagestate immediately after the data is programmed. After a passage of anamount of time from when the data is programmed, the monitoring unit 420may count a number of multi-bit cells having the threshold voltageshigher than the first read voltage level. According to exampleembodiments, the monitoring unit 420 may calculate a number of multi-bitcells of the first reference voltage state from the counted number ofmulti-bit cells, and determine whether the calculated number ofmulti-bit cells of the first reference voltage state is equal to anumber of first multi-bit cells stored as the meta data.

When it is determined that the calculated number of multi-bit cells ofthe first reference voltage state is equal to the number of firstmulti-bit cells stored as the meta data, the monitoring unit 420 maydetermine that the first read voltage level is still valid.

When it is determined that the calculated number of multi-bit cells ofthe first reference voltage state is less than the number of firstmulti-bit cells stored as the meta data, the monitoring unit 420 may seta new first read voltage level lower than the first read voltage level.The monitoring unit 420 may count a number of multi-bit cells having thethreshold voltages higher than the set new first read voltage level.According to example embodiments, the monitoring unit 420 mayrecalculate a number of multi-bit cells of the first reference voltagestate from the counted number of multi-bit cells, and determine whetherthe recalculated number of multi-bit cells of the first referencevoltage state is equal to a number of first multi-bit cells stored asthe meta data.

Repeating the above-described process, the monitoring unit 420 maysearch for the new first read voltage level of enabling the number offirst multi-bit cells stored as the meta data to be equal to thecalculated number of multi-bit cells of the first reference voltagestate. The monitoring unit 420 may calculate a difference between theretrieved new first read voltage level and the first read voltage levelimmediately after being programmed, and acquire the first change amount310 of the threshold voltages of the first reference threshold voltagestate.

Similarly, the monitoring unit 420 may acquire the second change amount320 of the threshold voltages of the second reference voltage state andthe third change amount 330 of the threshold voltages of the thirdreference voltage state. The monitoring unit 420 may estimate a changeamount of the threshold voltages of the remaining five threshold voltagestates based on the acquired first change amount 310, the acquiredsecond change amount 320, and the acquired third change amount 330, andadjust the read voltage levels with respect to the remaining fivethreshold voltage states based on the estimated change amount.

The detection unit 440 may detect the threshold voltages of themulti-bit cells of the multi-bit cell array 410 using the read voltagelevels adjusted by the monitoring unit 420, and extract the data storedin the multi-bit cells from the detected threshold voltages.

Another example embodiment is described with reference to FIG. 1 again.A memory device according to another example embodiment (notillustrated) may include a multi-bit cell array and a data detectionunit.

The data detection unit may read first multi-bit cells of the firstthreshold voltage state 120 immediately after being programmed using afirst read voltage V1. The data detection unit may read second multi-bitcells of the second threshold voltage state 140 immediately after beingprogrammed using a second read voltage V2.

When an amount of time passes from a time when the data is programmed,the data detection unit may newly search for a read voltage level of thechanged first threshold voltage state 121 and the changed secondthreshold voltage state 141. The data detection unit may search for athird read voltage V3 optimized for detecting the first multi-bit cellsof the changed first threshold voltage state 121. The data detectionunit may search for a fourth read voltage V4 optimized for detecting thesecond multi-bit cells of the changed second threshold voltage state141.

The data detection unit may read the first multi-bit cells of thechanged first threshold voltage state 121 using the third read voltageV3, and read the second multi-bit cells of the changed second thresholdvoltage state 141 using the fourth read voltage V4. According to exampleembodiments, a relation of |V4−V2|>|V3−V1| may exist.

FIG. 5 is a flowchart illustrating a method of estimating acharacteristic of a multi-bit cell according to an example embodiment.

Referring to FIG. 5, in operation S510, the estimation method mayinclude selecting a plurality of reference threshold voltage states froma plurality of threshold voltage states corresponding to data stored ina multi-bit cell array.

In operation S520, threshold voltage change over time values of theplurality of reference threshold voltage states may be extracted.

In operation S530, threshold voltage change over time values of theplurality of threshold voltage states may be estimated based on theextracted threshold voltage change.

In operation S540, a read voltage of the multi-bit cell array may beadjusted based on the estimated threshold voltage change.

In operation S550, multi-bit data stored in the multi-bit cell array maybe read using the adjusted read voltage.

In operation S510, the plurality of threshold voltage states may bedivided into a low level group, a medium level group, and a high levelgroup based on the level of the threshold voltage of the plurality ofthreshold voltage states.

In operation S510, one threshold voltage state may be selected as thereference threshold voltage state from each of the divided groups.

According to example embodiments. reliable reference data, with respectto the change of the threshold voltage states, may be acquired byrespectively selecting one reference threshold voltage state from thelow level group, the medium level group, and the high level group basedon the level of the threshold voltage of the plurality of thresholdvoltage states.

In operation S510, the plurality of threshold voltage states may bedivided so that a number of threshold voltage states of the low levelgroup may be greater than a number of threshold voltage states of thehigh level group. For example, when eight threshold voltage statesexist, the plurality of threshold voltage states may be divided toinclude four threshold voltage states in the low level group, and toinclude two threshold voltage states in the medium level group, and toinclude two threshold voltage states in the high level group. When theplurality of threshold voltage states is divided as described above, aprobability that the threshold voltage states having the high thresholdvoltage are selected as the reference threshold voltage state mayincrease. Since a charge loss mechanism may be significant in themulti-bit cells having the high threshold voltages, the estimationmethod may have an advantage if the estimation method extracts thresholdvoltage change information about the multi-bit cells having the highthreshold voltages.

In operation S510, the estimation method may select the second thresholdvoltage state, the fifth threshold voltage state, and the sevenththreshold voltage state as the reference threshold voltage states fromthe lowest threshold voltage state of eight threshold voltage states,and may select the sixth threshold voltage state, the seventh thresholdvoltage state, and the eighth threshold voltage state as the referencethreshold voltage state.

In operation S530, the estimation method may generate an approximationmodel with respect to the extracted threshold voltage change over timevalues of the plurality of reference threshold voltage states using alinear approximation scheme or a polynomial approximation scheme withrespect to the extracted threshold voltage change. According to exampleembodiments, in operation S530, the estimation method may reflect theextracted threshold voltage change on a predetermined approximationmodel and specify a coefficient of the approximation model using a curvefitting scheme.

The methods of estimating the characteristics of the multi-bit cellsaccording to example embodiments may be recorded in computer-readablemedia including program instructions to implement various operationsembodied by a computer. The media may also include, alone or incombination with the program instructions, data files, data structures,and the like. The media and program instructions may be those speciallydesigned and constructed for the purposes of example embodiments, orthey may be of the kind well-known and available to those having skillin the computer software arts. Examples of computer-readable media mayinclude, for example, magnetic media such as hard disks, floppy disks,and magnetic tape; optical media such as CD ROM disks and DVD;magneto-optical media such as optical disks; and hardware devices thatare specially configured to store and perform program instructions, forexample read-only memory (ROM), random access memory (RAM), flashmemory, and the like. Examples of program instructions may include bothmachine code, such as produced by a compiler, and files containinghigher level code that may be executed by the computer using aninterpreter. The described hardware devices may be configured to act asone or more software modules in order to perform the operations ofexample embodiments.

Flash memory devices and/or memory controllers according to exampleembodiments may be embodied using various types of packages. Forexample, the flash memory devices and/or memory controllers may beembodied using packages, for example Package on Packages (PoPs), BallGrid Arrays (BGAs), Chip Scale Packages (CSPs), Plastic Leaded ChipCarrier (PLCC), Plastic Dual In-Line Package (PDIP), Die in Waffle Pack,Die in Wafer Form, Chip On Board (COB), Ceramic Dual In-Line Package(CERDIP), Plastic Metric Quad Flat Pack (MQFP), Quad Flatpack (QFP),Small Outline Integrated Circuit (SOIC), Shrink Small Outline Package(SSOP), Thin Small Outline (TSOP), Thin Quad Flatpack (TQFP), System InPackage (SIP), Multi Chip Package (MCP), Wafer-level Fabricated Package(WFP), Wafer-Level Processed Stack Package (WSP), and the like.

The flash memory devices and/or the memory controllers may constitutememory cards. In this case, the memory controllers may be constructed tocommunicate with an external device for example, a host using any one ofvarious types of interface protocols, for example a Universal Serial Bus(USB), a Multi Media Card (MMC), a Peripheral ComponentInterconnect-Express (PCI-E), Serial Advanced Technology Attachment(SATA), Parallel ATA (PATA), Small Computer System Interface (SCSI),Enhanced Small Device Interface (ESDI), and Integrated Drive Electronics(IDE).

The flash memory devices may be non-volatile memory devices that canmaintain stored data even when power is cut off According to an increasein the use of mobile devices such as a cellular phone, a personaldigital assistant (PDA), a digital camera, a portable game console, andan MP3 player, the flash memory devices may be more widely used as datastorage and code storage. The flash memory devices may be used in homeapplications such as a high definition television (HDTV), a digitalvideo disk (DVD), a router, and a Global Positioning System (GPS).

A computing system according to example embodiments may include amicroprocessor that is electrically connected with a bus, a userinterface, a modem such as a baseband chipset, a memory controller, anda flash memory device. The flash memory device may store N-bit data viathe memory controller. The N-bit data is processed or will be processedby the microprocessor and N may be 1 or an integer greater than 1. Whenthe computing system is a mobile apparatus, a battery may beadditionally provided to supply operation voltage of the computingsystem.

It will be apparent to those of ordinary skill in the art that thecomputing system according to example embodiments may further include anapplication chipset, a camera image processor (CIS), a mobile DynamicRandom Access Memory (DRAM), and the like. The memory controller and theflash memory device may constitute a solid state drive/disk (SSD) thatuses a non-volatile memory to store data.

The foregoing descriptions of example embodiments have been presentedfor purposes of illustration and description. They are not intended tobe exhaustive or to limit example embodiments to the precise formsdisclosed, and obviously many modifications and variations are possiblein light of the above teaching.

Example embodiments having thus been described, it will be obvious thatthe same may be varied in many ways. Such variations are not to beregarded as a departure from the intended spirit and scope of exampleembodiments, and all such modifications as would be obvious to oneskilled in the art are intended to be included within the scope of thefollowing claims.

1.-4. (canceled)
 5. A device comprising: a multi-bit cell array; and acontroller configured to estimate a first threshold voltage change of aplurality of threshold voltage states corresponding to data stored inthe multi-bit cell array based on a second threshold voltage change ofat least one reference threshold voltage state selected from theplurality of threshold voltage states, wherein the second thresholdvoltage change is based on a difference between a threshold voltage whendata is programmed and a threshold voltage after a passage of an amountof time from when the data is programmed.
 6. The device of claim 5,wherein the controller is configured to extract the second thresholdvoltage change of the at least one reference threshold voltage state bymonitoring at least one threshold voltage of at least one multi-bit cellcorresponding to the at least one reference threshold voltage state. 7.The device of claim 5, wherein the multi-bit cell array is configured tostore a number of multi-bit cells programmed to correspond to each ofthe at least one reference threshold voltage state, and the controlleris configured to extract the threshold voltage change of the at leastone reference threshold voltage state based on the stored number of themulti-bit cells.
 8. The device of claim 7, wherein the controller isconfigured to store the number of multi-bit cells programmed tocorrespond to each of the at least one reference threshold voltage statein the multi-bit cell array.
 9. The device of claim 7, wherein the atleast one reference voltage state includes at least two referencevoltage states and the controller is configured to select a highestthreshold voltage state and a lowest threshold voltage state from amongthe plurality of threshold voltage states as two of the at least tworeference threshold voltage states.
 10. The device of claim 5, whereinthe controller is configured to estimate the threshold voltage change ofthe plurality of threshold voltage states using a characteristic changemodel acquired experimentally in advance
 11. A device comprising: amulti-bit cell array; and a detection unit configured to extract datastored in the multi-bit cells using read voltage levels of a pluralityof threshold voltage states corresponding to the data stored in themulti-bit cell array, wherein the detection unit is configured to adjustthe read voltage levels of the plurality of threshold voltage statesbased on a threshold voltage change of at least one reference thresholdvoltage state selected from the plurality of threshold voltage states,the threshold voltage change being based on a difference between athreshold voltage when data is programmed and a threshold voltage aftera passage of an amount of time from when the data is programmed.
 12. Thedevice of claim 11, further comprising: a monitoring unit configured tomonitor the threshold voltage change of the at least one referencethreshold voltage state by monitoring threshold voltages of at least onemulti-bit cell corresponding to the at least one reference thresholdvoltage state.
 13. The device of claim 12, further comprising: anestimation unit configured to estimate threshold voltage changes of theplurality of threshold voltage states using a characteristic changemodel acquired experimentally in advance.
 14. The device of claim 11,further comprising: an estimation unit configured to extract thethreshold voltage change of the at least one reference threshold voltagestate based on a number of multi-bit cells programmed to correspond toeach of the reference threshold voltage states, wherein the multi-bitcell array stores the number of the multi-bit cells.
 15. The device ofclaim 14, wherein the estimation unit is configured to estimatethreshold voltage changes of the plurality of threshold voltage statesbased on the extracted threshold voltage change.
 16. The device of claim14, wherein the estimation unit is configured to estimate thresholdvoltage changes of the plurality of threshold voltage states based onthe extracted threshold voltage change and a number of writes anderasures from the multi-bit cell array.
 17. The device of claim 14,wherein the estimation unit is configured estimate threshold voltagechanges of the plurality of threshold voltage states based on theextracted threshold voltage change and an elapsed time after writing theplurality of threshold voltage states into the multi-bit cell array. 18.An operating method of a memory device, the method comprising:extracting a first threshold voltage change of at least one referencethreshold voltage state from a plurality of threshold voltage statesstored in a multi-bit cell array; sending the first threshold voltagechange to a estimation unit; estimating second threshold voltage changesof the plurality of threshold voltage states based on the receivedthreshold voltage change; reading data from the multi-bit cell arraybased on the estimated second threshold voltage changes.
 19. The methodof claim 18, wherein the threshold first voltage change and the secondthreshold voltage changes are respectively a difference between athreshold voltage when a data is programmed and a threshold voltageafter a passage of an amount of time from when the data is programmed.20. The method of claim 18, wherein the estimating step includes using acharacteristic change model acquired experimentally in advance.
 21. Themethod of claim 18, wherein the estimating step is based on the firstthreshold voltage change and a number of data read operations performedon the multi-bit cell array.
 22. The method of claim 18, wherein theestimating step is based on the first threshold voltage change and anumber of data write and erasure operations performed on the multi-bitcell array.
 23. The method of claim 18, wherein the estimating step isbased on the first threshold voltage change and an elapsed time afterwriting the plurality of threshold voltage states into the multi-bitcell array.
 24. The method of claim 18, wherein the reading stepincludes adjusting read voltage levels based on the estimated secondthreshold voltage changes.